1. Field of the Invention
This invention relates to hydrogen absorbing alloy powders for use in the negative electrodes of alkaline rechargeable (secondary) batteries and processes for producing the same.
2. Description of the Related Art
For purposes associated with performance improvements including, for example, a reduction in the size and weight of electronic information equipment, a reduction in the size and an increase in the power output of electric tools, and a reduction in the size and weight of electric cars, it is desired to reduce the size and weight of rechargeable batteries used therefor and improve the performance thereof. As a result, it has become essential to improve the performance characteristics of rechargeable batteries, such as initial activity, a high rate discharge property and life characteristics.
However, in rechargeable batteries used for these purposes and, in particular, increasingly widely used nickel-metal hydride alkaline rechargeable batteries, their initial activity and a high rate discharge property and their life characteristics run counter to each other owing to the properties of the hydrogen absorbing alloy used as the negative electrode active material constituting the batteries. In order to make them consistent with each other, various attempts have been made from the viewpoint of alloy composition and alloy preparation, but no satisfactory result has been obtained.
As a means for improving the initial characteristics and a high rate discharge property of nickel-metal hydride alkaline rechargeable batteries, it is conceivable to reduce the hydrogen absorbing alloy powder constituting the negative electrode. To this end, various investigations on the particle size or particle diameter of the hydrogen absorbing alloy powder are made, for example, in Japanese Patent Provisional Publication Nos. 2-306541, 3-116655 and 4-292860. Although it is described therein to reduce the particle size or particle diameter, these techniques are less than practicable from the viewpoint of battery characteristics, production process, and handling of hydrogen absorbing alloys.
As to the addition of an electrically conductive material to the negative electrode of a nickel-metal hydride alkaline rechargeable battery, it is common practice to add the electrically conductive material, together with a binder and a thickener, during the preparation of a paste. Processes in which an electrically conductive material is mixed with an alloy powder are proposed in Japanese Patent Provisional Publication Nos. 6-44965 and 8-213005. However, since the alloy powder has a large particle size and is ground in a solvent system, these techniques are less than practicably from the viewpoint of battery characteristics and production process.